TrigTauRec: Standardisation of plot titles and labels

In order to achieve a uniform appearance as well as clarification of the plots, several changes to plot titles and axis labels were done.

Changes

• Changes Y-axis labels to "nEntries"
• Standardise use of uncapitalised, spelled out greek letters
• Distinguish tauMonitoringCaloOnlyMVA and tauMonitoringPrecisionMVA in plot titles
• Clarify the subtraction for plotted differences between quantities in labels and titles
• Correct spelling mistakes

Tagging @iriu, @ademaria and @gipezzul