README: Fix invalid escape sequence in string

The README contains an example of converting a string literal to a
regex, but the string literal contains an invalid escape sequence: "\."

Of course, this is a legal regex pattern, but in order to silence the
warning, use "\\." instead. The pattern matches as it does without the
extra escape.
Merge pull request #68 from modocache/fix-readme-regex-warning

README: Fix invalid escape sequence in string