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/python-modules/matplotlib/default.nix

@@ -55,7 +55,7 @@ buildPythonPackage rec {
   '';
 
   meta = with stdenv.lib; {
-    description = "python plotting library, making publication quality plots";
+    description = "Python plotting library, making publication quality plots";
     homepage    = "http://matplotlib.sourceforge.net/";
     maintainers = with maintainers; [ lovek323 ];
     platforms   = platforms.unix;