doc: fix latex formatting in cis docstring (JuliaLang#52845)

Author: stevengj

base/complex.jl

@@ -570,7 +570,7 @@ end
 """
     cis(x)
 
-More efficient method for `exp(im*x)` by using Euler's formula: ``cos(x) + i sin(x) = \\exp(i x)``.
+More efficient method for `exp(im*x)` by using Euler's formula: ``\\cos(x) + i \\sin(x) = \\exp(i x)``.
 
 See also [`cispi`](@ref), [`sincos`](@ref), [`exp`](@ref), [`angle`](@ref).