Fix misspellings and add units

Rochester/setDiffEQ5ModelingWith1stOrder/ur_de_5_12.pg  Fixed misspelling
ma123DB/set3/s7_5_4.pg  Fixed misspelling
OSU/high_school_apcalc/dchmwk8/prob21.pg  Added units
ASU-topics/setOptimization/4-5-41.pg  Added units
UVA-Stew5e/setUVA-Stew5e-C05S04-IndefInts/5-4-57.pg  Added units
FortLewis/Calc2/8-8-Probability/HGM5-08-08-08.pg  Changed "with" to "within"; added units
Dartmouth/setStewartCh18S3/problem_{1,2,4,5}.pg  Added units, fixed misspellings, etc.
Utah/AP_Calculus_I/set8_Exponentials_and_Logarithms/1220s6p4.pg
  Changed "degree" to "degrees"; revised hint and solution.