Fix warning about Unicode character ∘ (U+2218) in the PDF manual

* checking PDF version of manual ... WARNING
LaTeX errors when creating PDF version.
This typically indicates Rd problems.
LaTeX errors found:
! Package inputenc Error: Unicode character ∘ (U+2218)
(inputenc)                not set up for use with LaTeX.

See the inputenc package documentation for explanation.
Type  H <return>  for immediate help.
* checking PDF version of manual without index ... ERROR

Author: AlexisDerumigny

R/Faa_di_Bruno.R

@@ -22,7 +22,7 @@
 #'
 #' @returns a vector of size \code{length(x)} for which
 #' the \code{i}-th component is
-#' \code{(f ∘ g)^(k) (x[i])}
+#' \eqn{(f \circ g)^{(k)} (x[i])}
 #'
 #' @author Alexis Derumigny, Victor Ryan
 #'