Add blurbs for all concepts

[jmrunkle]

Also fix the typo in one of the concept exercise blurbs

Fixes #1897

pull request

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[ee7]

Nice.

I checked that there aren't any more blurbs with "TODO". With this PR (and typo fix), the full list of concept blurbs is:

$ git grep --break --heading 'blurb'

arrays/.meta/config.json
2:  "blurb": "Arrays are fixed-size collections of elements with the same type.",

basics/.meta/config.json
2:  "blurb": "Learn about Java basics like variable assignment and methods.",

booleans/.meta/config.json
2:  "blurb": "Booleans (true and false) are used for conditional logic.",

chars/.meta/config.json
2:  "blurb": "Strings are made up of chars (for example, a letter).",

classes/.meta/config.json
2:  "blurb": "Classes are the foundation of object-oriented programming in Java.",

conditionals-if/.meta/config.json
2:  "blurb": "Java supports various logical operations and conditionals like the if statement.",

constructors/.meta/config.json
2:  "blurb": "Constructors are used to create instances of a class.",

for-loops/.meta/config.json
2:  "blurb": "For loops allow for iteration over a specified range.",

foreach-loops/.meta/config.json
2:  "blurb": "For-each loops allow for iteration over every element in a collection.",

generic-types/.meta/config.json
2:  "blurb": "Generics types allow the compiler to enforce type restraints within a class or interface.",

inheritance/.meta/config.json
2:  "blurb": "Java supports single-parent inheritance for classes to extend and inherit behaviors.",

interfaces/.meta/config.json
2:  "blurb": "Java interfaces allow decoupling between a class and its implementation.",

lists/.meta/config.json
2:  "blurb": "Lists are an ordered collection that allows duplicates.",

numbers/.meta/config.json
2:  "blurb": "Java includes various numeric types including integer and floating-point numbers.",

strings/.meta/config.json
2:  "blurb": "Strings are an immutable sequence of Unicode characters.",

switch-statement/.meta/config.json
2:  "blurb": "Switch statements can switch between various execution paths.",

ternary-operators/.meta/config.json
2:  "blurb": "The ternary operator is a compact if-else statement.",

[ee7]

Also fix the typo in one of the concept exercise blurbs

If you wanted, you could also capitalize "atbash" in this blurb:

java/exercises/practice/atbash-cipher/.meta/config.json

Line 2 in c7b2205

"blurb": "Create an implementation of the atbash cipher, an ancient encryption system created in the Middle East.",

[jmrunkle]

If you wanted, you could also capitalize "atbash" in this blurb:

Punting on that one because the docs also have the same capitalization problem.