Captialize meta.description of all packages

In line with the Nixpkgs manual.

A mechanical change, done with this command:

  find pkgs -name "*.nix" | \
      while read f; do \
          sed -e 's/description\s*=\s*"\([a-z]\)/description = "\u\1/' -i "$f"; \
      done

I manually skipped some:

* Descriptions starting with an abbreviation, a user name or package name
* Frequently generated expressions (haskell-packages.nix)

pkgs/development/compilers/elm/packages/elm-repl.nix

@@ -24,6 +24,6 @@ mkDerivation {
   ];
   jailbreak = true;
   homepage = "https://github.com/elm-lang/elm-repl";
-  description = "a REPL for Elm";
+  description = "A REPL for Elm";
   license = stdenv.lib.licenses.bsd3;
 }