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.

Author: emanlove

src/Selenium2Library/keywords/_element.py

@@ -416,11 +416,11 @@ def press_key(self, locator, key):
         """Simulates user pressing key on element identified by `locator`.
 
         `key` is either a single character, or a numerical ASCII code of the key
-        lead by '\\'.
+        lead by '\\\\'.
 
         Examples:
         | Press Key | text_field   | q |
-        | Press Key | login_button | \\13 | # ASCII code for enter key |
+        | Press Key | login_button | \\\\13 | # ASCII code for enter key |
         """
         if key.startswith('\\') and len(key) > 1:
             key = self._map_ascii_key_code_to_key(int(key[1:]))