Fix random theme functionality of the [colorscheme] layer

[xudon9]

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Why this change is necessary and useful?

[Please explain in detail why the changes in this PR are needed.]

The random colorscheme functionality, uses a JSON key "frequency" according to Documentation.
However, in

SpaceVim/autoload/SpaceVim/layers/colorscheme.vim

Line 99 in e780e67

\ ctime - get(conf, 'last', 0) >= get(s:time, get(conf, 'fequecnce', ''), 0)

and in

SpaceVim/autoload/SpaceVim/layers/colorscheme.vim

Line 123 in e780e67

\ 'fequecnce' : s:random_frequency,

frequency or frequence is misspelled as fequecnce, causing the customized frequency not working.

And I noticed some inconsistency of using both "frequency" and "frequence", which is error-prone.

I fixed its functionality by changing all of them to "frequency".

[wsdjeg]

Merged, thanks.