Add edit task command into the user guide.

[Yskie]

Added edit task command into the user guide.

[codecov]

Codecov Report

Attention: Patch coverage is 86.70886% with 21 lines in your changes are missing coverage. Please review.

Project coverage is 79.88%. Comparing base (8668f89) to head (da32394).
Report is 42 commits behind head on master.

Files	Patch %	Lines
.../seedu/address/logic/commands/EditTaskCommand.java	87.32%	2 Missing and 7 partials ⚠️
src/main/java/seedu/address/model/TaskList.java	54.54%	3 Missing and 2 partials ⚠️
...a/seedu/address/model/person/UniquePersonList.java	57.14%	2 Missing and 1 partial ⚠️
...in/java/seedu/address/model/task/TaskPriority.java	84.61%	0 Missing and 2 partials ⚠️
.../seedu/address/logic/parser/AddressBookParser.java	0.00%	1 Missing ⚠️
src/main/java/seedu/address/model/Model.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #61      +/-   ##
============================================
+ Coverage     79.29%   79.88%   +0.59%     
- Complexity      642      682      +40     
============================================
  Files           102      104       +2     
  Lines          1980     2118     +138     
  Branches        186      215      +29     
============================================
+ Hits           1570     1692     +122     
- Misses          372      378       +6     
- Partials         38       48      +10     

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[breezetall]

LGTM!