Fixed constructor of RungeKutta with only one solver

[AlexandreMagueresse]

The former constructor had a typo. I also removed the symbol :FE in the docstring of EXRK_Euler_1_1.

[JordiManyer]

@AlexandreMagueresse can you pull master? I'll merge this, I guess it was forgotten...

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 88.72%. Comparing base (edd8cb7) to head (2c33fa7).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
src/ODEs/ODESolvers.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #999   +/-   ##
=======================================
  Coverage   88.72%   88.72%           
=======================================
  Files         181      181           
  Lines       23539    23539           
=======================================
  Hits        20886    20886           
  Misses       2653     2653           

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