Group: "Searching for keywords" searches for a single keyword ==> use singular

[mlep]

In the Add group window, among the five types of group, there is one called "Searching for keywords".
Since this type of group search for one keyword only, use of singular is more appropriate.

Note: help on groups is outdated. Will work on this later.

• Change in CHANGELOG.md described (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked documentation: Is the information available and up to date? If not created an issue at https://github.com/JabRef/user-documentation/issues or, even better, submitted a pull request to the documentation repository.

[koppor]

Still some localization check going wrong... Difficult thing with the localization:

  org.opentest4j.AssertionFailedError: Obsolete keys found in language properties file: 
  Please enter a field name to search for keywords.
  1. CHECK IF THE KEY IS REALLY NOT USED ANYMORE
  2. REMOVE THESE FROM THE ENGLISH LANGUAGE FILE
   ==> expected: <[]> but was: <[Please enter a field name to search for keywords.]>
      at org.jabref.logic.l10n.LocalizationConsistencyTest.findObsoleteLocalizationKeys(LocalizationConsistencyTest.java:126)

  Test findMissingLocalizationKeys() FAILED (1.7s)

  org.opentest4j.AssertionFailedError: DETECTED LANGUAGE KEYS WHICH ARE NOT IN THE ENGLISH LANGUAGE FILE
  PASTE THESE INTO THE ENGLISH LANGUAGE FILE
  
  Please enter a field name to search for a keyword.=Please enter a field name to search for a keyword.
   ==> expected: <[]> but was: <[Please enter a field name to search for a keyword. (src/main/java/org/jabref/gui/groups/GroupDialogViewModel.java LANG)]>
      at org.jabref.logic.l10n.LocalizationConsistencyTest.findMissingLocalizationKeys(LocalizationConsistencyTest.java:114)

[mlep]

@koppor Thank you for rising this point. I have not been careful enough (I forgot to alter a string in the LN10 EN file). Hopefully, it is ok now. Let's see what the script says...