Fix prettification of \phi, \varphi, and \varsigma

* lisp/textmodes/tex-mode.el (tex--prettify-symbols-alist): Fix
  prettification of \phi, \varphi, and \varsigma.

Concretely, \phi's symbol was actually varphi, \varphi was missing, and
\varsigma was commented out with a FIXME that it was reversed with
\sigma which it was not.

lisp/textmodes/tex-mode.el

@@ -2991,7 +2991,7 @@ There might be text before point."
     ("\\sigma" . ?σ)
     ("\\tau" . ?τ)
     ("\\upsilon" . ?υ)
-    ("\\phi" . ?φ)
+    ("\\phi" . ?ϕ)
     ("\\chi" . ?χ)
     ("\\psi" . ?ψ)
     ("\\omega" . ?ω)
@@ -3380,10 +3380,11 @@ There might be text before point."
     ("\\u{i}" . ?ĭ)
     ("\\vDash" . ?⊨)
     ("\\varepsilon" . ?ε)
+    ("\\varphi" . ?φ)
     ("\\varprime" . ?′)
     ("\\varpropto" . ?∝)
     ("\\varrho" . ?ϱ)
-    ;; ("\\varsigma" ?ς)		;FIXME: Looks reversed with the non\var.
+    ("\\varsigma" ?ς)
     ("\\vartriangleleft" . ?⊲)
     ("\\vartriangleright" . ?⊳)
     ("\\vdash" . ?⊢)