Correction in Oblique Mercator Parameters: 'gamma' to 'gamma_c' in 002-rumination.md

[pbroboto]

Please update the parameter for the Oblique Mercator in 002-rumination.md from "gamma" to "gamma_c". Note that the example omerc ellps=evrstSS variant x_0=590476.87 y_0=442857.65 latc=4 lonc=115 k_0=0.99984 alpha=53:18:56.9537 gamma_c=53:07:48.3685 is correct. I was stuck on the incorrect manual for a few hours before I noticed the example.