Typo fixes for item assignment in vec4 and mvec4.

[aforren1]

For example, on the master branch:

import glm

x = glm.vec4()
x[3] = 1
print(x)
# vec4(            0,            0,            1,            0 )

y = glm.mat4(0)
y2 = y[0]
y2[3] = 1
print(y2)
# mvec4(            0,            0,            1,            0 )

[Zuzu-Typ]

Thank you very much!

I've made so many of these little, yet fatal mistakes.. I would never have spotted them on my own.

[aforren1]

No problem, thanks for all the work you've put into this package!