Clarify what rosdep install does

[sloretz]

This changes the help text of the rosdep install command.

I have not run rosdep with this change locally. I'm assuming it is safe because it is only changing a line in a multi-line string.

[tfoote]

The changes look good. It seems that yaml.load due to unsafe behavior is now hard failing in python3. re: #651 and the unchanged instances.

[codecov-io]

Codecov Report

Merging #670 into master will increase coverage by 0.87%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #670      +/-   ##
==========================================
+ Coverage   75.19%   76.06%   +0.87%     
==========================================
  Files          30       30              
  Lines        2890     2987      +97     
==========================================
+ Hits         2173     2272      +99     
+ Misses        717      715       -2

Impacted Files	Coverage Δ
src/rosdep2/main.py	48.49% <ø> (ø)	⬆️
src/rosdep2/platforms/debian.py	89.14% <0%> (+3.42%)	⬆️
src/rosdep2/platforms/redhat.py	92.59% <0%> (+5.67%)	⬆️

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