Follow fixed typo.

README.txt

@@ -22,7 +22,7 @@ Example buildout::
   recipe = slapos.recipe.build:cmmi
   url = http://prdownloads.sourceforge.net/libpng/zlib-1.2.5.tar.gz?download
   md5sum = c735eab2d659a96e5a594c9e8541ad63
-  slapos_promisee =
+  slapos_promise =
     directory:include
     file:include/zconf.h
     file:include/zlib.h
@@ -45,7 +45,7 @@ Example buildout::
   environment =
     CPPFLAGS=-I${zlib:location}/include
     LDFLAGS=-L${zlib:location}/lib -Wl,-rpath -Wl,${zlib:location}/lib
-  slapos_promisee =
+  slapos_promise =
     directory:bin
     dynlib:bin/file linked:libz.so.1,libc.so.6,libmagic.so.1 rpath:${zlib:location}/lib,!/lib
     directory:include
@@ -79,12 +79,12 @@ Example buildout::
     unpack(%(url_0), strip_path=True)
     execute('make')
     execute('make install DEST=%(location)s')
-  slapos_promisee =
+  slapos_promise =
     ...
 
   [multiarchitecture]
   recipe = slapos.recipe.build
-  slapos_promisee =
+  slapos_promise =
     ...
   x86 = http://host/path/x86.zip
   x86-64 =  http://host/path/x64.zip