Fixed documentation error with 'Press Key' keyword

Escaped double baskslash within keyword documentation. Apparently RF escapes
characters so when it sees the double backslash is interprets this as a single
backslah when indeed it should be read as a double backslash.
This fixes issues #200.
Merge branch 'doc-press-key' into documentation-release
Merge branch 'documentation-release' into doc-xpath