Update triangle.rst

[kalpit3d]

corrected description of a,b,c (sides of the triangle)

Addressed problem description

text description was incorrect

Solution description

added correct text

Preflight checklist

Put an x letter in each brackets when you're done this item:

• [ ] Code changes complete.
• [ ] Code documentation complete.
• [ ] Documentation for users complete (or not required, if user never sees these changes).
• [ ] Manual testing done.
• [ ] Unit-tests implemented.
• [ ] Ready for merge.