Fix code formatting in timers docs

[rahulporuri]

fixes #793

This PR fixes code formatting in timers docs. The issue was the use of : and :: before the code block declaration.

[codecov-io]

Codecov Report

Merging #794 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #794      +/-   ##
==========================================
- Coverage   40.80%   40.78%   -0.03%     
==========================================
  Files         508      508              
  Lines       27832    27832              
  Branches     4217     4217              
==========================================
- Hits        11357    11350       -7     
- Misses      15993    15996       +3     
- Partials      482      486       +4     

Impacted Files	Coverage Δ
pyface/ui/qt4/action/menu_bar_manager.py	77.77% <0.00%> (-16.67%)	⬇️
pyface/ui/wx/system_metrics.py	89.47% <0.00%> (-10.53%)	⬇️
pyface/ui/qt4/util/modal_dialog_tester.py	86.46% <0.00%> (-3.01%)	⬇️
pyface/ui/qt4/window.py	71.17% <0.00%> (-2.71%)	⬇️
pyface/ui/qt4/application_window.py	89.28% <0.00%> (-2.39%)	⬇️
pyface/ui/qt4/tasks/editor_area_pane.py	53.67% <0.00%> (-2.21%)	⬇️
pyface/ui/qt4/tasks/advanced_editor_area_pane.py	50.86% <0.00%> (-0.87%)	⬇️
pyface/ui/qt4/tasks/split_editor_area_pane.py	64.82% <0.00%> (-0.60%)	⬇️
pyface/ui/qt4/console/console_widget.py	28.86% <0.00%> (-0.21%)	⬇️
pyface/ui/qt4/code_editor/code_widget.py	43.97% <0.00%> (+0.42%)	⬆️
... and 6 more

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[corranwebster]

I am far from a master of English grammar, but I think a single colon at the end of the preceding paragraph is better than a dash.