Fix typo in exception message

[gremmyz]

Closes #192

[codecov[bot]]

Codecov Report

Merging #206 (60d32c6) into master (3386235) will increase coverage by 0.73%. The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #206      +/-   ##
============================================
+ Coverage     64.22%   64.95%   +0.73%     
- Complexity      666      674       +8     
============================================
  Files           100      100              
  Lines          2440     2440              
  Branches        364      364              
============================================
+ Hits           1567     1585      +18     
+ Misses          692      679      -13     
+ Partials        181      176       -5     

Impacted Files	Coverage Δ
...in/java/seedu/connectus/commons/core/Messages.java	0.00% <ø> (ø)
...ava/seedu/connectus/logic/commands/AddCommand.java	100.00% <ø> (ø)
...tus/logic/commands/DeleteTagFromPersonCommand.java	83.33% <100.00%> (+33.33%)	:arrow_up:

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