[BUGFIX] Bad English in system extensions: "screen shot"

According to wiktionary [1] the word "screenshot" is a compound
word and not two separate words.

[1] http://en.wiktionary.org/wiki/screenshot

Resolves: #48989
Releases: 6.2, 6.1
Change-Id: Id7b9aa7eb98c4a2c6b0d47bddfb7cb9d51580aec
Reviewed-on: https://review.typo3.org/21296
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Alexander Opitz
Tested-by: Alexander Opitz
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn