style: add a missing space to invoke_fn15's definition

[wackbyte]

Oops, I guess I left out a space in my last PR.

[codecov]

Codecov Report

Merging #132 into master will not change coverage by %.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #132   +/-   ##
=======================================
  Coverage   78.86%   78.86%           
=======================================
  Files         141      141           
  Lines        8754     8754           
=======================================
  Hits         6904     6904           
  Misses       1850     1850           

Impacted Files	Coverage Δ
crates/mun_runtime/src/lib.rs	83.54% <100.00%> (ø)

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